The invention relates generally to mitigating stress corrosion cracking of components exposed to high temperature water in a high temperature water system. More particularly, certain embodiments of the invention utilize the synergetic benefit of zinc and low corrosion potential in mitigating stress corrosion cracking in high temperature water systems.
In many applications, such as nuclear reactors, steam driven turbines, or water deaerators, high temperature waters adversely affect the structures by causing stress corrosion cracks, corrosion, erosion, and so forth. For example, high temperature waters may cause stress corrosion cracking (SCC) in materials, such as carbon steel, alloy steel, stainless steel, nickel-based alloys, cobalt-based alloys, and zirconium-based alloys. Stress corrosion cracking includes cracks propagated by static or dynamic tensile stresses acting in combination with corrosion at a crack tip. These stresses can result or originate from differences in thermal expansion or contraction between components, relatively high or varying operating pressures, or various processes performed during the manufacture or assembly of the components or system. For example, residual stresses often result from welding, cold working, and other thermomechanical metal treatments. Water chemistry, welding, heat treatment, and radiation may also increase the susceptibility of a metal or alloy component to stress corrosion cracking.
Stress corrosion cracking occurs at greater rates under various conditions, such as the presence of oxygen, high radiation flux, and so forth. In nuclear reactors like pressurized water reactor (PWR) and boiling water reactor (BWR), a high radiation flux causes radiolytic decomposition of the reactor water, this decomposition produces oxygen, hydrogen peroxide, short-lived radicals, and various oxidizing species. These products of radiolytic decomposition promote stress corrosion cracking in the various system components, such as pipes, pumps, valves, turbines, and so forth. Operating temperatures and pressure for a boiling water reactor are typically about 288° C. and about 7 MPa; and those for a pressurized water reactor (“PWR”) are about 320° C. and about 15 MPa. Thus, the chance for stress corrosion cracking in reactor components is heightened.
One method of mitigating stress corrosion cracking of susceptible material in boiling water reactor is through the application of hydrogen water chemistry (HWC), which involves the addition hydrogen gas to the reactor feedwater. Addition of hydrogen reduces the level of oxidizing species, such as dissolved oxygen and hydrogen peroxide, thereby reducing the stress corrosion cracking susceptibility. Unfortunately, the hydrogen water chemistry technique often demands large quantities of hydrogen, to effectively reduce the stress corrosion cracking susceptibility to acceptable levels in the various components. Hydrogen demand can be reduced by coating or alloying the components with a noble metal catalyst. Despite the reduced hydrogen demands, there exists more efficient means of reducing stress corrosion cracking in certain high strength materials, such as cold worked, precipitation hardened, or irradiated materials.
Therefore, there exists a need for new approaches to mitigate stress corrosion cracking and to reduce the operating dose rate in nuclear reactors.